Corrosion has long been a problem when certain metals or alloys are used in applications in which they come into contact with an aqueous medium. For example, in heat-transfer systems, such as those found in internal combustion engines, alcohol-based heat transfer fluids (i.e., antifreezes) can be very corrosive to the metal surfaces of the heat-transfer systems. Compounding this problem is that the corrosion is accelerated under normal engine operating conditions (i.e., high temperatures and pressures). Corrosion inhibitors have been used to address these problems by protecting the metal surfaces in industrial and commercial heating and/or cooling systems such as heat exchangers used in the automotive industry (e.g., radiators which control engine operating temperatures, condensers and evaporators in air conditioning systems, and heater cores which provide heat to passenger compartments).
Heat exchangers for cars and light trucks have traditionally been constructed of soldered brass. More recently, however, in order to meet new environmental standards mandating improved fuel economy and reduced emissions, automotive manufacturers have begun using more lightweight metals (e.g., aluminum). See, e.g., Ward's Auto World, p.22 (September, 1996); Ward's 1996 Automotive Yearbook, p.27 (58th ed. 1996). Accordingly, heat exchangers in cars and light duty trucks are now being constructed using aluminum components. Typical aluminum components in heat exchangers include the tubes through which the coolant passes and in which the heat exchange occurs, the fins that are between the tubes to transfer heat from the tubes to the outside air, the header plates that hold the tubes and fins in place, and, in the case of heater cores, the end tanks that transfer the fluid between the tubes and the engine. See, e.g., Fortin et al., "Aluminum Materials and Processes for Aluminum Heat Exchanger Applications," SAE Technical Paper No. 852228 (1985). However, aluminum surfaces are particularly susceptible to corrosion. See, Darden et al., "Monobasic/Diacid Combination as Corrosion Inhibitors in Antifreeze Formulations," Worldwide Trends in Engine Coolants, Cooling System Materials and Testing, SAE Int'l SP-811, Paper #900804, pp. 135-51 (1990) ("SAE SP-811").
During the assembly of automotive heat exchangers, the aluminum components are typically joined together by a brazing process which generally involves joining these aluminum-containing components with a brazing alloy (i.e., an aluminum alloy with a melting point substantially lower than that of the components to be joined). The aluminum components to be joined are "brazed" by holding them together in a jig with the brazing alloy adjacent to or between these components, then heating them in a furnace to a temperature that will melt the brazing alloy without melting the components. The brazing process is described in "Base Metals, Brazing Filler Metals and Fluxes," Aluminum Brazing Handbook, p. 24 (January 1990).
One of the problems with brazing aluminum is that aluminum is prone to oxidation. Aluminum oxidation is promoted by high temperatures and the presence of oxygen and water vapor and can reduce the strength and durability of a brazed joint. One brazing method which avoids this problem is the controlled atmosphere brazing ("CAB") process. The CAB process generally involves pre-assembly of the aluminum components in a jig, spraying of the pre-assembled components with a flux, and then followed by introduction into an inert and controlled atmosphere brazing oven. The CAB process is the preferred brazing method, as compared to other brazing methods that also avoid aluminum oxide formation (e.g., vacuum brazing), because it is cost effective and generally produces higher quality heat exchangers with reduced scrap rates. The CAB process is described in U.S. Pat. Nos. 5,422,191; 5,360,158; 5,333,777; 3,971,501 and 3,951,328; Cooke et al., "Furnace Brazing of Aluminum with a Non-Corrosive Flux," SAE Technical Paper No. 780300 (1978); Claydon et al., "Brazing Aluminum Automotive Heat Exchanger Assemblies Using a Non-Corrosive Flux Process," SAE Technical Paper No. 830021 (1983); D. L. Childree,"A New Al--Si--Li Filler Metal that Enhances Brazeability of High-Strength Alloys in CAB and Vacuum," SAE Int'l SP-1175, Technical Paper No. 9602447 (1996); Brazing Handbook, American Welding Society (1991).
In the CAB process, the flux, which typically consists of alkali metal or alkaline earth metal fluorides or chlorides, serves a number of important functions. These include removing the aluminum oxide coating present on the exposed metal and brazing alloy surfaces, preventing reformation of the aluminum oxide layer during brazing, and enhancing the flow of the brazing alloy.
It has been found that the use of this flux in heat exchangers assembled by the CAB process results in residual flux on the surfaces of the aluminum components. The aluminum surfaces that generally are exposed to the flux residue include the inside of the heat exchanger tubes, i.e., radiator and/or heater core. However, because the commonly-used fluoride-based fluxes are considered non-corrosive to aluminum, the presence of residual flux per se has not been considered a problem.
Aluminum heat exchangers manufactured by the CAB process, and thus containing flux-treated aluminum surfaces, are fairly common in automotive and light truck applications and are currently in use by at least one major heat exchanger manufacturer. Ando et al., "Development of Aluminum Radiators Using the Nocolok Brazing Process--Corrosion Resistance of New Aluminum Radiators by Applying a Nocolok Brazing Process," SAE Technical Paper No. 870180 (1987); Park et al., "New Vacuum Brazed Aluminum Radiators for Ford Light Trucks," SAE Technical Paper No. 860078 (1986). There have been no reported problems associated with corrosion of these heat exchangers when used with antifreeze formulations containing conventional corrosion inhibitors such as triazoles, thiazoles, borates, silicates, phosphates, benzoates, nitrates, nitrites and molybdates. However, such conventional corrosion inhibitors may have problems in some applications, including expense and inadequate long-term protection. See U.S. Pat. No. 4,946,616, col. 1, lines 31-45; U.S. Pat. No. 4,588,513, col. 1, lines 55-64; SAE SP-811, pp. 137-38. Accordingly, automobile manufacturers have begun using, and several now require, organic acid based (or extended life) corrosion inhibitors. A number of organic acid corrosion inhibitors have been described. See, e.g., U.S. Pat. Nos. 4,382,008, 4,448,702 and 4,946,616; see also, U.S. patent application Ser. No. 08/567,639, (U.S. Pat. No. 5,741,436) incorporated herein by reference.
However, several formulations comprising mono-carboxylic acid(s), even those previously demonstrated to be effective aluminum corrosion inhibitors, have been unable to adequately protect CAB manufactured (i.e., flux-treated) aluminum surfaces. Further, while several aluminum corrosion inhibitors are known, many of these corrosion inhibitors are undesirable or unacceptable for the organic acid based antifreeze formulations used today. For example, phosphate salts, known to inhibit aluminum corrosion, are unacceptable because they have been prohibited for use in organic acid formulations by a number of original equipment automotive manufacturers. See, e.g., General Motors Engineering Standards, "Long-Life Automotive Engine Coolant Antifreeze Concentrate--Ethylene Glycol," Specification No. GM 6277M; Ford Engineering Material Specifications, "Coolant, Organic Additive Technology, Concentrate," Specification No. WSS-M97B44-C; Chrysler Corporation Engineering Standards, "Engine Coolant--Glycol Type--Inhibitied--Production and Service Use," Standard No. MS-9769. Similarly, sodium silicate, also a known aluminum corrosion inhibitor, is prohibited by General Motors and Ford. Moreover, sodium nitrate, known to effectively prevent aluminum pitting corrosion in conventional coolant formulations, has been reported to be much less effective in preventing pitting-corrosion than organic acid based inhibitors. See U.S. Pat. No. 4,647,392, col. 12.
Accordingly, there remains a need for a method using mono-carboxylic acid-based corrosion inhibitors to protect flux-treated metal surfaces, especially those found in automotive heat exchangers.